1. Field of the Invention
This invention relates to a breathing circuit. More particularly, the invention relates to a coaxial tube type breathing circuit in an artificial respiratory therapy device.
2. Description of the Prior Art
Various types of breathing circuits are currently in extensive use. They include circuits connected to respirators (otherwise called ventilators) and used for respiratory care, anesthetic circuits used for administering an anesthesizing gas and oxygen gas, for example, to patients, and circuits used for administering medicines, oxygen, and the like for the purpose of inhalant therapy.
These breathing circuits share a basic construction which comprises an inhalant circuit for connecting a respirator, an anesthetizer, or the like to a tracheal catheter, an oral mask, etc. fixated on patients and an exhalant circuit.
In mechanical inhalation and exhalation through the medium of a breathing circuit or anesthetic respiratory circuit, it is generally held that, in due consideration of the water balance and the heat balance of a patient's body, the inhalant breath is desired to have a fairly high temperature as compared with the ambient air, i.e., a temperature not lower than 31.degree. C., though not higher than the temperature of the patient's lungs (38.degree. C. at maximum). Particularly when artificial respiration is continued for a long period, the inhalant breath is required to have a temperature in the range of 32.degree. to 35.degree. C. and a relative humidity of 100%. To give to the inhalant breath such temperature and humidity, a combination heater and humidifier is provided immediately in front of the breathing circuit.
One of the conventional breathing circuits, for example, has a construction wherein an inhalant tube is connected via a combination heater and humidifier to the outlet side of a respirator and an exhalant tube is connected to the inlet side thereof, the two tubes are connected to branched pipes of a Y-shaped connector, and this connector is extended to a tracheal catheter fixated in a patient mouth. Further, a nebulizer adapter to deliver such a medicine as bronchodilator or sputum dissolver in the form of aerosol into the air pipe is inserted halfway along the length of the inhalant tube and an exhalation valve is likewise inserted in the exhalant tube.
Since the breathing circuit of this construction has the inhalant tube and the exhalant tube formed separately of each other, it inevitably has a large overall size and is inconvenient to handle. Further in this breathing circuit, the inhalant gas which is warmed in advance in the combination heater and humidifier is sent through the inhalant tube to the patient's lungs. During its passage through the inhalant tube, the inhalant gas radiates heat and loses temperature. If the temperature of the combination heater and humidifier is kept higher than that of the lungs, the temperature of the inhalant gas often falls below 31.degree. C. at the outlet of the connector on the patient's side. To preclude the fall of temperature of the inhalant gas in the connector, there is adopted a device utilizing a heater inside the exhalant tube or on the outer periphery of the exhalant tube. This device has a disadvantage that the addition of the heater increases the cost of equipment, the temperature control of the heater is difficult, and the heater itself is not easily cleaned and sterilized.
Further, near the patient's mouth, two tubes are disposed through the medium of the connector. Especially when the inner pressure of the circuit and the temperature of the inhalant gas are monitored near the mouth, the various devices which are required for the monitoring complicate the portion of the construction close to the mouth to a great extent and annoy the patient notably. The devices themselves are handled with difficulty.
As breathing circuits mainly used for anesthesis, the Payne circuit and the F circuit have been known in the art (Japanese Published Unexamined Patent No. 150,893/1979), for example.
These circuits have a common construction wherein a straight inner tube and a usually corrugated outer tube are coaxially assembled in a two-wall structure, with the interior of the inner tube intended as an inhalant circuit and the space between the outer tube and the inner tube as an exhalant circuit. One end of the coaxial tube on the patient side is connected to a connector which is provided with an inlet and an outlet. The other ends of the outer tube and inner tube on the anesthetizer side are fastened to outlets of a manifold. Through the inner communicating hole and the outer communicating hole of the manifold, the anesthetic gas from the anesthetizer is circulated.
Owing to the use of the coaxial tube, the circuits of this class are easy to handle. Compared with the aforementioned system in which the inhalant tube and the exhalant tube are disposed as two independent passages, the circuit of interest permits a reduction in the heat loss of the inhalant gas because the outer tube exists around the periphery of the inner tube serving as a path for the inhalant gas and further because the exhalant gas flows in close contact with the periphery of the inner tube.
This circuit is provided with neither an exhalation valve nor a nebulizer. Owing to the use of the coaxial tube, it does not permit insertion of such devices at points falling halfway along the length of the duct. It has a disadvantage, therefore, that it limits the types of respirators which are effectively usable with the circuit.
As described above, the inhalant gas which is delivered to the patient's lungs is desired to be at a temperature falling below the temperature of the lungs, preferably in the range of 32.degree. to 35.degree. C. In the case of a circuit which uses a coaxial tube constructed as described above and which has a combination heater and humidifier connected to the coaxial tube, the inhalant gas which has been treated by the combination heater and humidifier and readied for delivery to the lungs is likewise desired to have a temperature below the temperature of the lungs. This temperature condition counts much because the possibility of the temperature of the inhalant gas being abruptly raised by an unexpected trouble, the temperature being gradually heightened during a prolonged delivery of the inhalant gas, and the inhalent gas being delivered to the lungs at a temperature higher than the temperature of the lungs will be completely eliminated when the condition is fulfilled. Besides, since the difference between the temperature of the inhalant gas and the room temperature is narrowed, the loss of heat or the value of temperature drop is decreased and the amount of moisture suffered to form dew is also decreased.
It has been ascertained, however, that when a coaxial tube is formed in an ordinary structure and an inhalant gas heated by a combination heater and humidifier to a temperature lower than the temperature of the patient's lungs is forwarded so that, upon arrival at the patient's mouth, the gas may have a temperature in the range of 32.degree. to 35.degree. C., the circuit of interest fails to give a perfect solution to the problem of heat loss which the inhalant gas undergoes on route to the patient's mouth.
To be more specific, the magnitude of heat loss involved in the circiut of interest is decisively small as compared with the circuit using two independent tubes and it is also small as compared with the circuit involving delivery of the inhalent gas at a temperature higher than the temperature of the lungs. The decrease in the magnitude of heat loss, however, is not necessarily quite satisfactory. When the length of the coaxial circuit is increased, for example, the temperature drop of the inhalant gas during its travel through the circuit to the patient's mouth is increased. When the circuit is used for a prolonged period of artificial respiration, the amount of moisture suffered to form dew is also increased.
Moreover, the temperature range in which the combination heater and humidifier is allowed to be heated for the purpose of delivering to the patient the inhalant gas at an optimum temperature of 32.degree. to 35.degree. C. is so narrow that when the temperature of the combination heater and humidifier is varied, there is a possibility that the inhalant gas will be delivered to the patient's mouth at a temperature falling outside the optimum range.
Japanese Published Unexamined Patent No. 150,893/1979 discloses an embodiment wherein an inner tube is semi-fixed inside the portion of a connector on the patient side through the medium of a spacer. It has been ascertained that in this embodiment, the aforementioned disadvantage is not overcome because the inner tube comes off the spacer and the entire circuit elongates when the inevitably increased resistance of the gas passage and decrease of lung compliance cause the inner pressure of the circuit to rise to the order of more than some tens of cm of H.sub.2 O column.
Further in the circuit of the construction described above, since the inner tube used therein is a normally straight tube, the gas passage enclosed therein may possibly be collapsed heavily by a bend in the tube.
Furthermore, for the sake of the breathing circuit, it is desirable for a nebulizer to be inserted in the inhalant circuit as described above. For the nebulizer to retain its efficiency intact in the delivery of aerosol, it is desired to be disposed at a position closer toward the patient. In the circuit of the construction described above, since the circuit is formed in a coaxial structure as indicated above, it is difficult for the nebulizer to be integrally inserted and disposed completely within the circuit. No construction incorporating a nebulizer within a breathing circuit has ever been developed to date.
British Patent Publication No. 2,029,703A discloses an anesthetic circuit wherein an inner tube and an outer tube which are both corrugated tubes are coaxially assembled in a two-wall structure and both the inner and outer tubes are fixed simultaneously to a connector on the patient side. In the circuit of this construction, collapse of the circuit does not frequently occur because the inner tube has a corrugated wall.
In this case, as concerns the overall elongation of the circuit as a whole, since both the outer tube and the inner tube are fixed to the connector on the patient side, the inner tube offers resistance to and restricts the elongation of the outer corrugated tube when the inner pressure of the circuit is increased. Compared with the circuit of the previous construction, therefore, this circuit has lower overall elongation and less dead space. A coaxial tube constructed by coaxially assembling two corrugated tubes of different diameters in accordance with the disclosure of British Patent Publication No. 2,029,703A mentioned above and in conformity to the conventional technique has shown a still large overall circuit elongation and, particularly for use as a breathing circuit, has proved unsatisfactory in terms of dead space and constancy of performance.
Even in the circuit of this construction, integral insertion of a nebulizer in a disposition capable of enhancing the efficiency of aerosol delivery is difficult. This insertion has not yet been materialized in any of the ducts developed to date.
An object of this invention, therefore, is to provide a novel breathing circuit.
Another object of this invention is to provide a breathing circuit of the construction of a coaxial tube which suffers from far less heat loss than the conventional circuit of the same function.
Yet another object of this invention is to provide a breathing circuit which has a simple, compact structure and an easily operated mechanism, entails no heavy heat loss, produces no large amount of moisture for dew formation, offers no heavy inhalant and exhalant resistance, and excels in the versatility of a respirator.